At present, there is a great need to identify and trace chemical contaminants, especially organic compounds, as they move through the environment. The monitoring of contaminants in ground water is of particular concern because of the difficulty of real time, in situ analysis that not only indicates the presence of, but can quantitate and identify contaminants and their products produced in situ. Further, there exists a need for real time monitoring of contaminants in the atmosphere. It would be convenient to have a monitoring system that is local to the site to be monitored but which can be read at a distance convenient to those regulating these contaminant emissions.
The Raman effects, including RRS, SERS, and SERRS, are well known in the scientific literature, particularly as described in Grasselli et al., Chemical Applications of Raman Spectroscopy, Wiley-Interscience, John Wiley and Sons, New York, 1981. The details of Raman spectroscopy are discussed in U.S. Pat. No. 3,556,659, which includes further references on Raman theory.
A number of devices exist that can be used for Raman spectroscopy. McLaughlan et al., in U.S. Pat. No. 4,573,761, disclose a fiber optic probe for Raman analysis comprising a bundle of optical fibers grouped such that at least one optical fiber is used exclusively for transmitting light into the sample while at least two optical fibers arranged at an advantageous angle with the transmitting fiber are used exclusively for collecting light from the sample.
Johnson et al., in J. Am. Chem. Soc., (1986) 108, 905-912, describe the use of ultraviolet resonance Raman techniques to characterize the photoionization products of phenol, tyrosine, and tryptophan.
Marley et al, (1985) Appl. Spectrosc. 39, 628-633, and (1984) Appl. Spectrosc. 38, 540-543, describe the use of Raman spectroscopy for trace analysis of phenols in water.
A major difficulty associated with Raman spectroscopy is the low intensity of the scattered light compared to the exciting light. Elaborate spectrometers, having high light gathering power and dispersion, high stray light rejection, and sensitive detector, are required to isolate and measure the low intensity Raman scattered light. These instruments are costly and sensitive, and thus are not well suited for use in commercial manufacturing or processing facilities. As a result, they have rarely been used outside of laboratory environments.
Another problem associated with Raman spectroscopy is that of fluorescence, which competes with the Raman effect. Many compounds, including natural products and minerals, fluoresce or emit light when exposed to laser light in the visible region, which further interferes with the Raman signal in samples. Fluorescence bands are usually broad, and are often featureless. Although fluorescence bands are often successfully used for quantitation and sensitive indication that a fluorescing compound is present, fluorescence bands do not provide the unambiguous fingerprinting quality of the Raman spectra. Because of its very nature, the Raman spectrometer also acts as a very sensitive fluorescence spectrometer, and as such, the Raman signal can be buried in the fluorescence.
Another disadvantage of Raman spectra is that, if there are several chemical compounds present in a mixture to be analyzed, all of the compounds will contribute a Raman signal. The resultant Raman spectrum will be an addition of all of the Raman spectra of all of the components in the mixture, and thus very complicated and potentially confusing.
A variety of applications of optical fibers to spectroscopic problems have been described in the past, with particular emphasis on UV-visible absorption and fluorescence techniques. Optical fiber-based sampling devices allow the sample to be remote from the spectrometer. While infrared absorption spectrometry can provide structural information, it is not amenable to fiber optic probes because of poor transmission of infrared light by glass or plastic fibers. Since Raman scattering spectrometry normally uses visible light which is efficiently transmitted by optical fibers, it can provide vibrational information about the sample yet still be easily coupled to a fiber optic probe.
Rong et al., in Anal. Chem. (1986), 58, 1116-1119, describe the use of colloidal silver sols for surface-enhanced Raman scattering.
Johnson et al., in Anal. Chem. (1984), 56, 2258-2261, disclose that ultraviolet resonance Raman spectroscopy could be used to detect polycylic aromatic hydrocarbons such as naphthalene, anthracene, and pyrene in solvents such as water and acetonitrile. Laser excited resonance Raman spectroscopy has been reported by Van Haverbeke et al. in Anal. Chem. 1979, 57, 932-936.
Chamberlain et al., in an NSF Report NSF-RA-760393 disclose that the use of a laser to measure the Raman spectra of molecular constituents remotely can be used to monitor gaseous pollutants.
Enlow et al., in Anal. Chem. (1986), 58, 1119-1123, disclose a method for detecting nitro polynuclear aromatic compounds by surface-enhanced Raman spectrometry. The substrates used were silver-coated substrates consisting of latex spheres on glass and filter paper, and prolate silicon dioxide posts on quartz.
There has been some activity in remote detection techniques using optical fibers to carry the Raman spectra to remote sites for detection and evaluation. For example, Chudyk et al., in Anal. Chem. (1985), 57, 1237-1242, describe a method of detecting groundwater contaminants using far-ultraviolet laser-induced fluorescence. Schwab et al., in Anal. Chem. (1984), 56, 2199-2204, disclose a fiber optic Raman probe wherein both the exciting laser light and the collected Raman scattering are conducted by optical fibers. Nguyen et al., in Analysis, (1986), vol. 14, No. 7, 334-343, disclose a method for selective cartography using Raman spectroscopy using optical fibers for remote and in situ analysis. Walragen et al., in Applied Spectroscopy, (1972), 26, 585-589, and Ross et al., in Applied Spectroscopy, (1981), 35, 438-442, disclose that Raman spectra can be intensified by using liquid core optical fibers.
Schwab et al., in Applied Spectroscopy, (1987), 41, 126-130, disclose a long path Raman cell (LPRC) for high sensitivity Raman spectroscopy.